The stages of winter dormancy for fruit trees

Fruit trees go through three physiological stages of dormancy: acclimation, endo- and eco-dormancy, and deacclimation.

Acclimation

There is a period of time when the tree stops growing and gets ready for freezing temperatures. This happens after the fruit has ripened and been harvested in late summer and early fall. For a fruit tree to become fully acclimated, there needs to be a gradual period of cooling before the first hard freeze. Some fruit species will also begin to acclimate as the days grow shorter. Full acclimation is usually reached in October and early November.

Some years, we have a long-warm fall followed by a sudden, hard freeze. This is a common cause of trunk splitting and other problems that may show up a year or two later. This year was a good fall for acclimation.

Endo- and eco-dormancy

Once fruit trees reach full dormancy, they enter the next stage of physiological dormancy. Endo-dormancy is often referred to as chilling hours accumulation period. Chilling temperatures are not freezing temperatures. The most efficient chilling temperatures are between 32 and 45 degrees. Below 32 and above 50, there is very little chilling accumulation. When temperatures go above 60 degrees during endo-dormancy, the chilling hours are actually reversed. So, on days when the temperature never goes above freezing, the trees do not accumulate chilling hours.

Southern states grow cultivars with low-dormancy requirements, because if the chilling requirements are not met, problems such as delayed foliation, reduced fruit set, and poor fruit quality can occur.

Once the chilling requirement is satisfied, the tree enters the eco-dormancy portion of dormancy. This is a period of time when the tree is just waiting for warmer temperatures to stimulate spring growth.

The reason people in eastern Idaho are encouraged to purchase fruit cultivars with high chilling requirements is because it is possible to meet low chilling hours before the harsh temperatures of December and January. Then, when there is a warm period in the middle of the winter or in early spring, the tree breaks dormancy and can be severely damaged by spring freezes. Select fruit cultivars with 700 – 1,000 hours chilling requirement.

Deacclimation

As temperatures reach above 40 after chilling requirements have been met, the trees will start to wake up and break dormancy. The flowering buds can still tolerate a fair amount of frost and light freezing. The most sensitive time is when the flowers are fully open. When there is little fruit on trees that flowered heavily, it is because either the temperatures dropped below the critical threshold, or there were few pollinators available during that critical time.

Next time you buy a fruit tree, ask the folks at the nursery about the chilling requirements of the cultivars that are available. At the very least, do some of your own research and select cultivars that not only taste good, but will be consistent at producing fruit year-after-year.